This invention relates to an artificial airway device to facilitate lung ventilation in an unconscious patient, and more specifically to such a device designed for placing in the oropharynx of the patient in order to prevent airway obstruction, to permit either spontaneous or controlled ventilation and to prevent the inhalation into the lungs of extraneous matter such as blood.
To maintain the airway of an unconscious patient, and to achieve the three objectives mentioned above, it is normal practice in general anaesthesia to use an endotracheal tube, which is a flexible tube of rubber or plastics, usually with an inflatable cuff around the distal end. Alternatively, an oro- or naso-pharyngeal airway may be used, which is a flexible tube extending from the mouth or nose into the pharynx but not into the larynx, and which, unlike the endotracheal tube, is used in conjunction with a face mask. While preventing obstruction of the airway by the tongue, the oro- or naso-pharyngeal airway cannot conveniently be used for controlled ventilation and does not prevent inhalation of extraneous matter.
The endotracheal tube is introduced through the larynx into the trachea or windpipe, whereupon the cuff is inflated through a small auxiliary tube to seal against the wall of the trachea. Introduction of the endotracheal tube is a skilled operation normally requiring use of a laryngoscope to guide the tube through the larynx, past the vocal cords and into the trachea. There is a risk that the tube or the laryngoscope may cause damage to soft tissues or to the sensitive structures of the larynx. It is not always possible to see the larynx, making intubation difficult or impossible in some patients. There can be a risk of accidental intubation of the oesophagus or of the right or left main bronchus. Placing of the tube in the trachea effectively narrows the interior passage or lumen of the trachea and provides a potential source of damage through infection or pressure while preventing normal upward flow of mucus from the trachea and rendering effective coughing impossible.
My British Patent Specification No. 2111394B describes an artificial airway device comprising a curved or flexible tube and a mask portion carried at one end of the tube. The mask portion has a flexible annular peripheral formation which may be inflatable and which surrounds a hollow interior space or lumen of the mask portion. Said annular peripheral formation of the mask portion is pre-formed with a roughly elliptical shape so as to be capable of conforming to, and of fitting readily within, the actual and potential space behind the larynx, thereby forming a seal around the 3circumference of the laryngeal inlet without having the device penetrate into the interior of the larynx. The tube opens into the lumen of the mask portion to provide the airway, with the axis of the tube at an acute angle to the general plane of the annular peripheral formation and substantially aligned with the major axis of the roughly elliptical shape of the annular peripheral formation of the mask portion. The device thus constitutes a laryngeal mask.
This device has proved successful in use. Insertion of the device has been found to be easy and convenient in the majority of patients. A laryngoscope is not usually required. The mask does not enter the larynx or trachea, so the risk of damage to these structures is avoided and the tracheal lumen is not narrowed, as is the case upon insertion of an endotracheal tube. The risk of accidental entry into the oesophagus or one of the main bronchi is also avoided. Once in place, the laryngeal mask generally permits the lungs to be ventilated by positive pressure. Alternatively, the patient may be permitted to breathe spontaneously.
An improved laryngeal mask is described in my Patent No. GB 2,205,499B, in which there are provided two flexible cross-bars extending across the aperture in the peripheral annular flexible formation. These bars preferably extend substantially parallel with the major axis of the generally elliptical annular formation. As this patent explains, the parallel bars serve to prevent the epiglottis from blocking the airway passage of the mask.
In the existing laryngeal mask, the peripheral seal formation is an inflatable annulus which, when inflated, is a generally elliptical toroid. Its manufacture presents difficulties, in that the process uses a so-called "floating core" within the outer or female part of the mould. This core is supported by a relatively small pin which ultimately defines the bore of the inflation inlet but which must maintain uniform spacing from the adjacent wall surface of the female mould. It is desired that the wall of the moulded annulus be uniformly thin.sup.1, for example, a selected single thickness in the range 0.4 to 1.0 mm, preferably 0.7 mm. In practice, however, injection of liquid rubber into the mould is under high pressure, and the "floating core" support pin is not capable of resisting small displacements which can cause thickness variations in the wall of the moulded product. In consequence, it has been necessary to maintain a relatively thick wall in the peripheral annular formation so that small changes in wall thickness produced by movement of the core during injection will not result in deformity or rupture of the device when air-inflated in the course of use. The difficulty with a relatively thick wall is that the inflatable annulus is less compliant in adapting itself to the contours of the human pharynx and larynx, thus reducing the efficacy of the desired seal to the laryngeal inlet, and/or unduly limiting the pressure which it is possible to use for inflation of the lungs without loss of seal. FNT .sup.1 Thin, for greatest flexibility in establishing the intended peripheral seal to the laryngeal inlet; and uniformly thin, to avoid weak areas that might invite herniation.
A further disadvantage of the existing manufacturing process is that it is highly labor-intensive, since the tubular airway portion of the device to which the peripheral annular formation is attached must be connected via a third part, or back plate. This third part is separately moulded and forms an appropriate shape at one end to fit within the peripheral annular formation and at the other end to receive the tubular airway portion of the device. Finally, the back plate and the cut inner periphery of the inflatable ring must be so adhesively secured as to complete the inflatable integrity of the ring as well as the sealing effectiveness of the back plate to the inflatable ring.